Optical Coherence Tomography (OCT) is a technique of optical imaging developed recently. It is based on the technique of Optical Coherence Domain Reflectometry (OCDR). The output of a low coherence light source comprises temporal coherence, which interferes in an interferometer and forms interference patterns. According to the interference patterns, an image of the sample can be obtained.
The longitudinal resolution of the image is proportional to the bandwidth of the light source with equation: lc=0.441×(λ02/Δλ).
Wherein, lc is the coherence length of the light source namely the longitudinal resolution of the image. λ0 is the center wavelength of the light source, and Δλ is the bandwidth of the light source.
Theoretically, the wider the bandwidth of the light source is, the shorter the interference patterns are, and the higher the resolution of the image is.
OCT is a technique that can obtain the 3-dimensional high resolution image of a sample. Wherein, the technique of OCDR is used to have the longitudinal resolution, and a combination of lenses is used to raise the transverse resolution of the image.
Referring to FIG. 1, there is shown an interferometer configuration in accordance with the traditional OCT technique. A light beam 121 from a low coherence light source 12 is collimated by a collimating lens 141 and then input into a beam splitter 14. The light beam 121 is split to a sample light 123 and a reference light 125.
The sample light 123 is focused to a sample 17 by a focusing lens 143. A portion of the sample light 123 is reflected or scattered by the sample 17 and collimated by the focusing lens 143 then reflected by the beam splitter 14 and focused to an image sensor 181 by a focus lens 147. According to the difference of the light paths of the sample light 123 and the reference light 125, interference patterns are formed on the image sensor 181. By using a signal processor 183 to analyze the signal detected by the image sensor 181, the information of the sample can be shown on a display 185.
Furthermore, there is a longitudinal scanning unit 161 driving the mirror 145 to move up and down for adjusting the light path of the reference light 125, and the longitudinal scan (in depth) of the sample can be achieved. A transverse scanning unit 163 is used to drive the sample light 123 to scan on the surface of the sample 17 in 2 transverse dimensions.